The present invention relates to a method for measuring a level of a surface of molten metal poured into a mold when molten metal is continuously cast. Also, the present invention relates to a continuous casting apparatus using the same therefor.
In general, when molten metal is continuously cast, powder is supplied onto an upper face of a pool of molten metal poured into a mold and then melted by heat of molten metal. Melted powder flows into between mold walls, which are oscillated perpendicularly, and a solidified shell of metal, which is drawn out at a predetermined rate, by a relative motion caused between the mold walls and the solidified shell. A meniscus and an end of the solidified shell are deformed by dynamic pressure generated when molten metal flows into between the mold walls and the solidified shell. This deformation is repeated at a period of the oscillation given to the mold. Therefore, periodic wrinkles, which are called oscillation marks, are formed on a surface of a cast slab. When depths of the wrinkles are large, defects will be caused on the surface of the cast slab. Depending upon a type of steel, only a specific element is segregated at bottom portions of the oscillation marks, or a large number of blowholes are generated and further a quantity of inclusion in the metal is increased, which lowers the yield of products.
On the other hand, when billets or cast bloom, the sectional areas of which are small, are continuously cast, rape seed oil is used instead of the above powder. This rape seed oil burns on the meniscus and becomes graphite, so that it can prevent the solidified shell from seizing onto the mold walls. However, it is difficult to form clear oscillation marks at regular intervals on the surface of the cast bloom. Therefore, stability of continuous casting work and quality of the cast bloom are inferior to those of the method in which powder is used.
Concerning the method for controlling initial solidification , Japanese Unexamined Patent Publication (Kokai) No. 52-32824 discloses the following technique. In the continuous casting method, in which molten metal is poured into a water-cooled mold, which is oscillated periodically, together with lubricant and drawn out downward continuously, there is provided an electromagnetic coil round the mold, and an alternating current is continuously fed to the electromagnetic coil, so that an alternating magnetic field can be formed. An electromagnetic force generated by the alternating magnetic field is given to the meniscus of molten metal poured into the mold. Therefore, the meniscus is curved by the action of the electromagnetic force, so that the surface property of the cast slab can be improved. Further, Japanese Unexamined Patent Publication (Kokai) No. 64-83348 discloses the following technique. When an electromagnetic force is given to molten metal in a mold by an electromagnetic coil, an alternating magnetic field is given like pulses. Due to the foregoing, the electromagnetic force is intermittently given to molten metal in this powder casting method, so that the surface property of the cast piece can be more improved. Furthermore, Domestic Re-publication of PCT International Publication No. 8-805926 discloses the following continuous casting method. In order to give an electromagnetic force, an amplitude of an alternating current is changed in the same manner as that of the frequency of oscillation given to a mold. When frequency (fm) of the mold and frequency (fp) of the alternating magnetic field are set to satisfy the expression 0.69xe2x89xa6ln(fp/fm)xe2x89xa69.9, wherein in is a constant, the meniscus is stably generated, and the surface property of the cast slab can be more stabilized.
Concerning the method for detecting a level of molten metal, various methods have been developed and carried out. Examples of the method for detecting a level of molten metal are: a conventional method in which a float is used, optical method (photoelectric transfer method), method in which ultrasonic waves or radioactive rays are used, method in which dipped electrodes are used, method in which a thermocouple is used and method of an electromagnetic induction system. Further, as disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 3-122526, 3-138536, 4-187355 and 4-238661, a method is developed, in which a change in impedance of a receiving coil caused by a change in an eddy current is measured as a change in phase. Japanese Unexamined Patent Publication (Kokai) No. 4-238661 discloses a method in which a changing range of a molten metal surface level is divided into a plurality of portions and measured by a plurality of detectors, and when the detectors are changed over, the molten metal level can be continuously measured.
However, when the above prior art is actually applied to a continuous casting apparatus into which an electromagnetic induction heating device is incorporated, it is impossible to accurately detect a surface level of molten metal in a mold. When the accuracy of detection of a molten metal surface level is deteriorated, it becomes difficult to control the molten metal surface level. Therefore, it is difficult to stably form a meniscus of molten metal. As a result, the surface property of a cast slab is deteriorated.
On the other hand, as a molten surface level detecting means used for a continuous casting apparatus, an eddy current type molten metal surface level meter is frequently used, to which an electromagnetic induction phenomenon generated by an alternating magnetic field is applied as described above. Other than the above eddy current type molten metal surface level meter, there are provided a method in which a thermocouple embedded in a mold is used and a method in which a permeability type sensor for xcex3 rays is used. However, from the viewpoints of improving the measuring accuracy and response property, the eddy current type sensor is the best sensor. Therefore, the eddy current type sensor is widely used in the process of continuous casting. However, as long as the electromagnetic coil is used in the continuous casting apparatus, an alternating magnetic field generated by the electromagnetic coil acts as noise on the eddy current type molten metal surface level sensor. Therefore, it is impossible to accurately detect a molten metal surface level, and further the measuring accuracy is deteriorated, which is a big problem. This problem is specifically described as follows. For example, when the eddy current type sensor is used in a magnetic field of continuous electromagnetic induction casting in which the intensity is not less than 1000 gauss and the frequency is 200 Hz, the signal output voltage is saturated, and it becomes impossible to conduct measurement. In a magnetic field of pulse electromagnetic induction casting, the signal output voltage is saturated when the magnetic field is turned on. Therefore, it is impossible to measure a molten metal surface level in this saturated condition, which is a serious problem in the measurement of a molten metal surface level.
In order to solve the above problems, the present inventors have paid attention to the following points. In a continuous casting apparatus in which the surface property of a cast slab is improved by generating a pulse magnetic field, a high intensity noise acts on the eddy current type molten metal surface level sensor in a period of time in which pulses are given (the magnetic field is turned on), however, noise seldom acts on the eddy current type molten metal surface level sensor in a period of time in which pulses are interrupted (the magnetic field is turned off). From the above viewpoint, the invention has been accomplished as follows. The period of time in which the magnetic field is turned on and the period of time in which the magnetic field is turned off are periodically repeated and inputted into a signal processing device of the eddy current type molten metal surface level meter or mold-built-in type level meter. In this signal processing device, the molten metal surface level is detected only in the period of time in which the magnetic field is turned off, and the molten metal surface level is not detected in the period of time in which the magnetic field is turned on. Due to the foregoing, it becomes possible to detect a molten metal surface level stably and accurately without being affected by the noise of the magnetic field generated by the electromagnetic coils. The summary of the invention will be described as follows.
(1) A continuous casting apparatus for continuously casting molten metal comprising:
an electromagnetic coil arranged round molten metal in a mold in such a manner that the electromagnetic coil surrounds the mold;
an electric power supply unit for periodically giving a case in which a root-mean-square value of a single phase alternating current is high and a case in which a root-mean-square value of a single phase alternating current is low, to the electromagnetic coil;
a sensor system for detecting a surface of molten metal; and
a medium for transmitting information to the sensor system in the case where a root-mean-square value of a single phase current periodically generated by the electric power supply unit is high and also transmitting information to the sensor system in the case where a root-mean-square value of a single phase current periodically generated by the electric power supply unit is low.
(2) A continuous casting apparatus for continuously casting molten metal according to item (1), wherein a value of a molten metal surface level generated by the sensor system is determined by information of the medium for conducting transmission.
(3) A continuous casting apparatus for continuously casting molten metal according to item (1) or (2), wherein a value of a molten metal surface level generated by the sensor system is outputted as information of the medium for conducting transmission only when the alternating current of a single phase is low.
(4) A continuous casting apparatus for continuously casting molten metal according to one of items (1) to (3), wherein the medium for conducting transmission is an electric signal wire.
(5) A continuous casting apparatus for continuously casting molten metal according to one of (1) to (4), the sensor system including:
a primary signal generating device;
a primary sensor coil for generating an electromagnetic field from a current of the primary signal;
a secondary sensor coil for generating a secondary voltage when the secondary sensor coil crosses the electromagnetic field; and
a secondary signal processing device for processing a signal of the secondary voltage generated by the secondary sensor coil.
(6) A continuous casting apparatus for continuously casting molten metal according to one of items (1) to (5), wherein an output of the primary signal processing device is turned on and off according to information of the medium for conducting transmission.
(7) A continuous casting apparatus for continuously casting molten metal according to one of items (1) to (5), wherein an output of the secondary signal processing device is turned on and off.
(8) A continuous casting apparatus for continuously casting molten metal according to one of items (1) to (7), wherein an output of the electric power supply unit is zero when an intensity of a single phase alternating current of the electromagnetic coil is low.
(9) A continuous casting apparatus for continuously casting molten metal according to one of items (1) to (8), wherein one period To is 30xe2x89xa6To xe2x89xa6300 (mxc2x7sec) where To is one period in the case where an intensity of a single phase alternating current of the electromagnetic coil is high and low, and a period of the single phase alternating current f is 60xe2x89xa6fxe2x89xa6400 (Hz).